Development in information processing technology in recent years involves the use of optical recording media as recording media having a small size and a large capacity. Such optical recording media are classified into recording media exclusively used for reproduction such as CD and CD-ROM, write-once type recording media on which information can be written only once, and rewritable type recording media on which information can be rewritten. Known rewritable recording media include magneto-optical recording media such as magneto-optical disks. Recording is performed on such a magneto-optical recording medium by applying an external magnetic field while irradiating a recording layer (magnetic layer) with a laser light beam so that the polarity of magnetization in the recording layer is inverted into a direction of the external magnetic field.
It is also one of important technical tasks for the magneto-optical recording medium to achieve a high recording density. A system for recording signals converted into multi-valued information has been hitherto known as a means for achieving a high density on a magneto-optical recording medium, as described in, for example, Proceedings of 13th Congress of Applied Magnetics Society (published in 1989, page 63) and Japanese Journal of Applied Physics, Vol. 28 (1989) Supplement 28-3, pp. 343-347.
In such a multi-valued recording system, a plurality of magnetic layers having mutually different coercive forces are stacked, and magnetization of a specified magnetic layer is selectively magnetically inverted by modulating, at multiple levels, the intensity of a magnetic field applied to the magnetic layers. It is described that four-valued (quaternary) signal recording can be achieved in accordance with this system by providing, in a recording medium, three magnetic layers having mutually different coercive forces
However, according to the multi-valued recording system applied to the magneto-optical recording medium as described above, in order to detect, upon reproduction, signals recorded as multi-valued signals, the multi-valued signals have been distinguished by slicing, at a plurality of levels, signals detected from the magneto-optical recording medium. Accordingly, it has been impossible to obtain a large difference in signal amplitude corresponding to each of multi-valued states, and it has been difficult to clearly distinguish two states with a close difference in signal amplitude therebetween. For this reason, a problem arises in that the S/N ratio is low with respect to reproduced multi-valued signals. Therefore, it has been demanded to realize a reproduction technique for obtaining reproduction signals at a high S/N ratio from a certain magneto-optical recording medium subjected to high density recording.
As for magneto-optical recording media each having a plurality of magnetic layers, a recording medium, in which information can be recorded and reproduced independently on each of magnetic layers, may serve as an extremely effective recording medium when various types of information are recorded on a single recording medium in a correlated manner, or when they are simultaneously recorded and reproduced in parallel together with a plurality pieces of channel information.
JP-A-4305841 discloses a method for reproducing a multilevel information which has been recorded by illuminating with a recording light a magneto-optical recording medium, which has a magneto-optical layer wherein a magnetization direction may be changed by means of a magnetic field and heat with the illumination of the light, and an enhanced layer wherein a refractive index with respect to a reproducing light may be changed depending on a heating condition with the illumination light, to independently vary the magnetization direction of the magneto-optical recording layer and the refractive index of the enhanced layer. In the method, the information is reproduced by illuminating the magneto-optical recording medium, onto which the multilevel information has been recorded, with two reproducing lights having different wavelength in order to independently detect the intensity of the respective lights. The enhanced layer must have a characteristic that the refractive index with respect to an illumination light used for reproduction is changed depending on a heating condition based on illumination of the light.
JP-A-7147027 discloses a method for reproducing a multilevel information from a magneto-optical recording medium having a first magnetic recording layer and a second magnetic recording layer by using two laser beams with length .lambda.1, .lambda.2. The magneto-optical recording medium needs to use a spacer layer for transmitting the laser beam having the wavelength .lambda.2 and reflecting the laser beam having the wavelength .lambda.1.